1. Field of the Invention
The present invention relates to a method of manufacturing multiple-core complex material and multiple-core clad complex material used for composing heat-radiating substrates for use with semiconductor integrated circuits.
2. Description of Related Art
Generally, for composing heat-radiating substrate material for semiconductor elements and semiconductor integrated circuits there have been required to have those advantages such as heat-radiating characteristic (thermal conductivity), satisfactory thermal expansion coefficient approximating those of the package composing material of semiconductor elements and circuit substrates, light-weight construction, satisfactory mechanical workability, and inexpensiveness. To satisfy these requirements, either the assembly or the complex material composed of copper (Cu), molybdenum (Mo), tungsten (W), Ni-Fe alloy, Ni-Co-Fe alloy, aluminum (Al), and silicon (Si) has been known.
Nevertheless, as a whole, the assembled material for composing heat-radiating substrate is not widely used because of such disadvantages as (trouble to assemble) several kinds of materials for its users, unlikeliness to lower the thermal resistance of the assembled material below a certain level as a result of assembling several kinds of materials in composition, and unlikeliness to enlarge their junction area because each material has its own thermal expansion coefficient different from each other, thus causing the substantial restriction to incur on the design, production, and thermal resistivity of its package.
Reflecting those disadvantages mentioned above, heat-radiating substrates are mostly composed of the complex material today. The complex material is manufactured by applying either the sheet-cladding process or the powder-sintering process.
The sheet-cladding process heaps up such material with substantial thermal conductivity and thermal expansion coefficiency as copper and the like and such sheet material with less thermal conductivity and thermal expansion coefficient as Ni-Fe alloy and Ni-Co-Fe alloy so as to form not less than double layers by bonding them together with pressure. Though the sheet-cladding process permits mass production of the complex sheet material on the industrial basis to yield large-area substrates inexpensively, the complex material becomes anisotropic, on the contrary, and difficult to be bonded together so that this problem should be taken into consideration. The problem is that its thermal conductivity tends to be higher in the direction parallel the surface but lower in the direction of the substrate thickness, whereas in the bonded multiple-layer sheet material, thermal expansion coefficient is smaller in the direction along the surface but larger in the direction of the substrate thickness. Accordingly, this process is unfavorable since semiconductor (devices being equipped on the multiple-layer sheet material is cracked) or peeled to result in leakage which possibly causes the semiconductor devices themseleves to be malfunctional or broken. On the other hand, there are two kinds of the powder-sintering process including the blended metallic powder-sintering process and the infiltrating process. The blended metallic powder sintering process is to blend metal powder (like the combination of copper with tungsten) simultaneously with the sintering process, solidifying the blended metallic powder into the complex sheet material. If the blend ratio of the copper powder were identical to that of the tungsten powder, thermal expansion coefficient of the complex sheet material grows not less than that being yielded with the infiltrating process which will be described later, and thus, blending of copper and tungsten at the equal blend ratio is not desirable. The infiltrating impregnating process is to press and sinter pulverized tungsten powder to form a porous body with uniformly and randomly distributed fine holes and to infiltrate molten copper into the porous body before eventually forming the complex material by integrating the porous body with copper. The molten-copper infiltrating process is disclosed in the Japanese Patent Application Laid-Open No. 59-141247. According to this art, appropriate thermal expansion coefficient is yielded by causing tungsten to constrain the thermal expansion of copper. Futhermore, since the copper-tungsten complex material available for heat-radiating substrate has low rate of thermal expansion and high thermal conductivity, as a whole, reflecting these desirable characteristic properties, the heat-radiating substrate material is manufactured by applying the molten-copper infiltrating process today. However, application of this process has disadvantages that such material (costs expensive) and that it has specific gravity and hardness to make the machining work difficult, and that is has limited uses. The Japanese Patent Application Laid-Open No. 62-294147 proposes the art of manufacturing alloy material which substantially coincides the thermal expansion coefficient of the heat-radiating substrate material with that of semiconductor elements by properly adjusting the blend ratio of tungsten or molybdenum, copper and nickel. However, those materials with high melting points such as molybdenum and tungsten are so expensive and so difficult to be processed that utilization of the heat-radiating substrate material for the semiconductor integrated circuits mainly composed of those materials with high melting points results in such disadvantages as it being expensive and difficult to be processed.
To avoid those disadvantages mentioned above and improve thermal conductivity, Japanese Patent Application Laid-Open No. 61-208899 proposes materials for composing heat-radiating substrates. Likewise, Japanese Patent Application Laid-Open No. 63-14829 proposes a method of manufacturing heat-radiating substrates. These prior arts propose the material for composing heat-radiating substrates made of a plurality of metal-wound wire material with low thermal expansion coefficient and the method of manufacturing it, respectively. The heat-radiating substrate material proposed by those inventions cited above decreases the amount of molybdenum to be consumed. Although its cost can be reduced to a certain extent, its cost economy cannot yet fully be achieved. Furthermore, since there is substantial difference of the thermal expansion coefficient in the direction of its axial line and in the orthogonal direction against the axial line, reliability of the product is still insufficient.
Recently, the method of restraining heat evolution and cooling a variety of substrates used for ICs, LSIs, and VLSIs, is a critical issue among the concerned. Manufacturers have employed such a method of cooling semiconductor-element substrates as causing metallic material to be bonded with a variety of substrates for radiating heat from them. However, utilization of the conventional material for radiating heat from the substrates mentioned above results in problems in that it (costs are expensive) and in that the material cannot easily be processed. On the other hand, application of inexpensive material is inappropriate owing to the presence of anistropy in the thermal expansion coefficient and thermal conductivity, owing to its large specific gravity, and poor adjustment with elements and ceramics when circuit assembly is underway.